In European Patent Application No. 91306517 published on Jan. 22, 1992 under Publication No. 0467673A2, an elevator active suspension system was disclosed showing a method and apparatus for actively counteracting a disturbing force acting horizontally on an elevator platform moving vertically in a hoistway. In particular, as it relates to the invention disclosed herein, a control scheme is shown in FIG. 69 thereof for a pair of active guides. Even more particularly, a centering control is shown for counteracting low frequency, relatively large forces tending to de-center, shift, cant or otherwise tip the car from a steady horizontal position as it moves up and down the hoistway. A similar centering control is shown in FIG. 9 of U.S. Pat. No. 5,117,946. Both of the above-mentioned documents disclose inventions assigned to assignee hereof.
The details of an exemplary elevator horizontal suspension is shown in the above-mentioned U.S. Pat. No. 5,117,946 particularly at column 5, line 56 and concluding at column 6, line 66, which is hereby incorporated by reference along with the related Figures in the drawing thereof.
Also in both of the above-mentioned documents, an elevator horizontal suspension is shown on opposite sides of an elevator car. Both front to back and side to side suspension controls are considered, the front-to-back control being much simpler than the side-to-side control on account of a need, in the side-to-side control, for comparing the position of the car with respect to both rails on opposite sides of the hoistway and centering the car with respect thereto. Two separate front to back controls can be handled independently, if desired, with respect to the separate rails.
During extensive testing of the system disclosed in the above-mentioned documents it was found that the filters and compensation were be difficult to select in a way that permitted the control to be fast enough to satisfy the objectives for quickly centering the car yet slow enough to maintain stability. It was also found that, for a geared motor control that was back-driveable, there was an undesired back driving phenomenon when forces were sufficient to overcome the rather weak control provided in background for the homing control, primarily for the actuator not being utilized. The intent of the homing control was to maintain a constant preload on the unselected roller. In other words, while one of the actuators was actively centering the car the other we designed to be returned to a home position using a weak control primarily intended for use when the actuator was not being utilized to center the car but nonetheless left on all the time.